The present invention relates to a boresight alignment method for aligning optical sighting systems with a laser which may provide such functions as rangefinder, designator, or target illuminator requiring accurate positioning of the beam at a distant point.
One of the major potential sources of error in laser targeting or tracking systems involves the boresight misalignment between the laser being directed at the target and the sighting system being used to aim the laser at the target. Modern military vehicles employ electro-optical fire control systems using multiple sensors to detect and track desired targets. The multiple sensors may include a visible sensor, a forward-looking infrared sensor, and an intensified night vision sensor, all of which may be disposed within the same instrument as the laser. In order to meet laser targeting performance goals, the boresight accuracy along the various sensors is required to be of high accuracy, e.g. 100 microradions. Conventional alignment systems for use with multiple sensors integrated into a single system may have poor boresight co-alignment among the sensors which results from a number of factors associated with the use of wholly or partially separate optical systems held in position by mechanical mountings with less than perfect stability. These systems are generally costly to build and align due to the reliance on precision fixturing, equipment, and assembly processes to produce the initial alignment setting which is often assumed to be maintained throughout the life of the system. The initial alignment settings however do not always maintain their settings throughout the life of the system. In fact, periodic boresight alignments need to be performed in cases such as when a laser optical system is utilized under rugged field conditions in which thermal or structural perturbations are likely to misalign optical sensors and laser(s) relative to each other. It is an object of the present invention to provide an improved boresight target alignment which allows viewing and rapid alignment of optical systems to a laser with high accuracy under rugged military or industrial field operating conditions.
In the past, field collimators for laser systems have taken two forms, ground systems and airborne systems. Ground systems (e.g., G/VLLD, MULE, PAL) have assumed that laser and day sight alignment are maintained with sufficient accuracy over differing environments, and have provided collimation systems that only align the day and night channels. This may be accomplished by centering a wire cross hair structure in the day sight and, using heating and/or emissivity effects, while viewing the cross-hairs against their natural background. The principle disadvantages of this class of systems is that they do not provide validation of the laser alignment directly to either beam line and the operator is obliged to move a sighting feature in the collimator to establish a reference line-of-sight. This can be particularly onerous, given that achievement of high accuracies in collimators always requires ensuring that they are very rigid structures and that they are not perturbed during use.
Airborne systems often use targets which interact with the laser beam to render it visible to one or more of the onboard sensors. In some cases, a frequency conversion process creates a green light which can be seen in the system day sight. Unfortunately, extension of this technique to a night sight requires the addition of a movable, heated aperture or cross hair, much as in a ground system. In the other cases, the target absorbs the laser energy in the focal plane and the temperature rises locally in the region of the beam, allowing it to be viewed by the thermal imager in the system. Unfortunately, a visible reticle or spot has to be coaligned by the operator in this case to allow use of this technique to align with the day channel.
By comparison with current practice, the present invention allows the use of a single reflective collimator to align many different sights in different spectral bands by simply backlighting a pinhole, burned by the laser in a foil or membrane located at the collimator focal point, with diffuse, incoherent illumination in all the desired bands. No additional adjustments or alignments within the collimator are needed once the pinhole has been burned. The simplicity, inherent mechanical stability, and lack of adjustments allow the key concepts of the invention to be embodied using collimator designs employing very high performance optical systems, e.g. high-magnification telephoto designs of short physical dimensions and lightweight systems using diamond machining of all components from a single alloy.
Prior art U.S. patents for boresighting of laser designation systems can be seen in U.S. Pat. No. 4,422,758 to Godfrey et al. for a boresighting of airborne laser designation systems. In this system, a laser beam is focused onto a refractory target in the boresighting device creating very briefly an incandescent hot spot which can be seen by all three sensors. Radiation from this hot spot is collimated by the boresight device optics and projected back into the sights exactly anti-parallel to the laser beam. By aligning the sight reticles with this hot spot, all three types of sights are aligned relative to the laser. This technique is limited, however, to operation with relatively high average power, high pulse rate or CW lasers to provide the desired heating and prevent problems seeing the beam in scanning thermal imaging sensors. In the prior U.S. patent to Hatfield, Jr., U.S. Pat. No. 5,025,149, an integrated multispectral boresight target generator combines visible and infrared sensors and a laser designator. A pinhole is illuminated with a multiband source to produce a single visible and infrared target which is detected by visible and infrared sensors. A pair of beam splitters and an associated corner reflector define the parallelism of the visible and infrared radiation transmitted from the multiband source along an optical axis. A reflective telescope is used to project the visible radiation to the visible sensor and the infrared radiation in conjunction with a periscope to the infrared sensor. The requirement to align the pinhole mechanically with the laser beam renders it of little use in a rugged field environment where the system is likely to be used by unsophisticated operators.
In contrast to these prior patents, the present invention allows the viewing and aligning of one or more optical systems to a laser with high accuracy under rugged military or industrial field operating conditions by using the laser designator to burn a pinhole in a foil or film located at the focal position of a collimator, which pinhole is subsequently backlit and viewed through the collimator by the optical sights to which the alignment is to be accomplished. The only operator actions required are to line up the individual sensor reticles to the illuminated spot, rather than the multiple adjustments required using prior techniques.